What the process capability study has to do with six sigma
The Process Capability study is typically performed for two reasons:
The process is expected to meet the customer requirements, specifications, or product tolerances. The process relies on a statistical analysis of variation of the process output compared to the allowable specification links. There are 2 forms of Process studies, the Process Potential Study (Ppk) and the Process Capability Study (Cpk).
The Ppk is used to define what has already happened in a single lot of product. Ppk is used in the early phases of a project to obtain an estimate of process variation. It provides a guideline to follow.
The Cpk process is a sampling of a continuous process over planned intervals. It is basically a prediction of what is likely to occur in the future.
Typically the studies results are shown in control charts or as a single number expressed using a process capability index. Both the control charts and the index charts require running the process enough to obtain measurable output to show the process is stable and variability can be reliably estimated.
Six Sigma is primarily focused on eliminating defects to improve all process to a level of 3.4 defects per one million opportunities.
Six Sigma asserts the following:
Process Capability Study and Six Sigma
The Process Capability study shows that if process changes are implemented at the right time for the right reasons will have a positive outcome on the product. If done during the study, a process change only introduces error and uncertainty. After the study is completed, you can then introduce changes to improve the process. The end result of the study is designed to meet customer expectations, specifications, and product tolerances.
This is unlike Six Sigma which is a data-driven, systematic approach to problem solving. Six Sigma has the customer in mind at all times and continually looks for ways to improve the overall satisfaction of the customer from the beginning.